1. Field of the Invention
The present invention relates to a circuit design, and more particularly, to a short circuit protection of a pulse width modulation (PWM) circuit.
2. Description of the Related Art
A PWM controller is usually applied to a switching regulator. As shown in FIG. 1, the PWM controller transforms an input voltage Vin into a stable output voltage Vout.
To prevent the output voltage Vout from being accidentally grounded (short circuited), in addition to the voltage transformation module 104 installed inside the PWM controller 108, there is also a short circuit protection design. Generally, the short circuit protection design uses a short circuit protection module 106 to detect a feedback pin FB. When the output voltage Vout is in short circuit, the power transistor 102 is turned off, and the off state is forwarded to the voltage transformation module 104 to signal the occurrence of a short circuit. In addition, the sudden voltage drop at the feedback pin FB causes the PWM controller 108 to be temporarily turned off. However, with such type of short circuit protection mechanism, devices in relevant paths and the PWM controller will still be damaged by an overlarge current.
FIG. 2 shows another type of prior art short circuit protection design. In such a design, a P-type MOSFET transistor 202 is used to prevent the system power VDDS from being affected by the output voltage Vout. In addition, the short circuit protection module 206 also detects an under voltage lock out (UVLO) signal. If the UVLO signal indicates that the input voltage VDD of the PWM controller 208 is too low, a PRDB pin sends out a gate control signal to make the transistor 202 temporarily turn off. However, in this structure when the transistor 202 is set to be turned off, the VDD voltage recovers the original voltage level to disable the indication of the UVLO signal that indicates the input voltage VDD is too low, which causes the PRDB pin to turn the transistor 202 on. The repeated on/off states of the transistor 202 cause the VDDS voltage to fail to remain at a stable voltage level and instead the voltage will follow a triangular waveform until the release of the short circuit of the output voltage Vout. Furthermore, the short circuit protection mechanism is not initiated upon the initiation stage of the PWM controller. As such, the prior short circuit protection fails to provide full protection to the circuit.
Therefore, there is an important need for a new design of PWM controller to increase the efficiency of the short circuit protection.